Variables that Influence Measured Concrete Compressive Strength

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Variables that Influence Measured Concrete
Compressive Strength

• Heather J. Sauter, E.I.T.
• Ph.D. Candidate

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Acknowledgement

• The following paper was referenced throughout
  this presentation
• Richardson, David N., Review of Variables that
  Influence Measured Concrete Compressive
  Strength, NAA Circular No. 132, NRMCA
  Publication No. 179, National Ready Mixed
  Concrete Association, Silver Spring, MD 20910.

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Why concrete strength is tested

• In research, physical laws or properties may be
  under investigation
• During construction, an estimate of the in-place
  strength of concrete may be desired for
  determining the safe time to strip forms or to
  proceed with further work.
• The adequacy of mix proportions may need to be
  verified.
• Compressive strength data is necessary for
  quality control.

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Testing of Cylinders

• Hardened concrete is typically evaluated for
  acceptance using 6 in. (152 mm) by 12 in. (305
  mm) cylinders.
• The measured results are dependent upon adhering
  strictly to standardized uniform procedures.
• Most testing errors produce lower strength
  results.

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Testing of Cylinders

• The consequences of falsely low results can be
• Unnecessary delays
• Costly follow up testing
• Wasteful overdesign
• Possible rejection of good concrete

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Variables

• Sampling
• Casting
• Initial Curing
• Transporting
• Laboratory Curing
• Capping
• Testing
• Reporting

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Sampling

• Proper batch representation by sampling is often
  not achieved.
• ASTM C172 specifies that the sample should be
  taken from atleast two places in the middle
  portion of the load.
• Remixing of the sample is also specified to
  ensure uniformity of the sample.
• Maximum time interval between sampling and
  casting is also specified.
• Make sure that the concrete for a set of
  cylinders comes from a single truck.

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Casting

• Consolidation
• Mold Material
• Specimen End Condition
• Cylinder Uniformity

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Consolidation

• To get the proper consolidation, the method of
  consolidation should match the material being
  consolidated.
• The proper number of layers, consolidation
  effort, rod type, and mold type are all
  important.
• Insufficient consolidation can lead to a strength
  loss of as much as 61.

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Mold Material

• The type of mold material is important in terms
  of rigidity, water absorption, and expansion.
• A more rigid mold will flex less during
  consolidation resulting in a more compact
  specimen.
• Mold types
• Cardboard molds with steel base plate
• Steel
• Plastic

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Specimen End Condition

• The finished condition of the cylinder ends are
  important to concrete strength.
• A rough end can mean capping problems
• Air pockets trapped underneath a sulfur cap can
  lead to a loss of strength up to 12.
• The rough interface between the concrete and
  capping material will present a nonstandard
  stress distribution during testing

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Specimen End Condition

• Upon casting, the specimens should be left on a
  level, smooth surface.
• Cutting the cylinder end evenly may be a better
  procedure but it becomes very costly.

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Cylinder Uniformity

• Unusual fracture types have been noted for
  cylinders when rod penetration has been
  insufficient, resulting in poor bond between
  layers.

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Initial Curing

• Temperature
• Humidity
• Specimen Disturbance during Initial Curing

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Temperature

• Lower than standard curing temperatures for 3 to
  7 days can cause as much as a 7 loss in
  strength.
• One day of freezing followed by standard curing
  can lead to a loss up to 56.
• Higher than standard curing temperatures may
  increase early strength, but later strengths will
  suffer.

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Humidity

• Insufficient humidity during initial curing can
  lower measured strength.
• Proper humidity can be approached by covering the
  cylinders.
• In general, the cylinders should be brought into
  the lab within 24 hours of casting.

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Specimen Disturbance during Initial Curing

• Rough treatment of the cylinder while it is
  undergoing setting and initial hardening can
  damage the specimen.
• Disturbance can be in the form of gross
  disturbance or vibration.
• For traffic induced vibrations, wet mixes can
  lose as much as 5 strength through the
  segregation mechanism, while stiff mixes can gain
  4 strength by virtue of improved consolidation.

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Transporting

• Rolling and bumping in the back of a pickup truck
  and result in a 7 strength loss.
• Dropping cylinders from waist level can lower
  strength by atleast 5.
• Cylinders should be cushioned during transport
  and handled gently at all times.

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Laboratory Curing

• Upon arrival to the lab, cylinders should
  immediately be stripped, logged in, and placed in
  a proper curing environment until the time of
  test.
• Temperature needs to be maintained at 73 /- 3 F
  either in a fog room or a lime-saturated water
  tank.
• The purpose of wet curing is to maximize
  hydration of the cement.

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Capping

• End Conditions
• Capping Materials

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End Conditions

• Four basic specifications per ASTM C617
• The capped ends must be flat within 0.002 in.
  (0.05 mm) in 6 inches.
• The ends must be approximately perpendicular to
  the specimen axis.
• The ends must be approximately parallel to each
  other.
• The caps must not be excessively thick.

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Capping Materials

• Any capping material used should not induce a
  stress distribution in the cylinder that would
  cause inaccurate measured strength results.

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Testing

• Moisture Condition and Temperature
• Loading Rate
• Specimen Misalignment
• Loading Platens
• Specimen Behavior
• Seating Behavior
• Machine Calibration
• Frame Stiffness
• Postfailure Inspection

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Moisture Condition and Temperature

• The cylinder must not be allowed to dry out after
  capping. A dry cylinder will exhibit 2-30
  higher strength than a saturated one.
• A recent study showed that the moisture gradient
  is important and that the surface drying causes
  shrinkage and lateral biaxial compression, which
  increases measured strength.

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Loading Rate

• Compressive strength increases with increased
  loading rate the relationship between strength
  and rate of loading is logarithmic.
• Choose a loading rate per ASTM C39 and repeat it
  to reduce variability.

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Specimen Misalignment

• A 12 loss in strength can result from eccentric
  loading.

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Loading Platens

• The degree of stiffness of the steel loading
  platens can affect the stress distribution within
  the test cylinder.
• Radius of the spherically seated head is
  important to provide sufficient restraint during
  loading and obtain good initial alignment.

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Loading Platens
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Specimen Behavior

• There are three basic types of specimen behavior
  during loading
• both ends pinned
• both ends fixed
• one end fixed, one end pinned

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Specimen Behavior

• Once significant loads are applied, any tilting
  will induce bending and nonuniform straining
  which will result in a loss of strength.
• The fixed end situation is desirable so that
  there is no tilting during loading.

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Seating Behavior

• Seating behavior is a function of lubricant type,
  sphere-seat contact area, contact type,
  sphere-seat surface finish, and platen geometry.
• It is recommended that high-pressure grease not
  be used.
• A thin coat of petroleum jelly has proven good
  performance. ASTM C39 allows the use of motor
  oil also.

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Machine Calibration

• A significant percentage of testing machines in
  use can fail to be within calibration standards
  giving strength differences of as much as 19.
• Accuracy of a machine can change over a
  relatively short time
• Care must be taken during calibration not to
  accidentally damage the platens.

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Frame Stiffness

• Machine frame stiffness can be important,
  especially for high strength concrete.
• A less rigid frame will stretch during loading
  storing more elastic energy.
• The less rigid machine will more closely follow
  the rapid deformation of the specimen and failure
  will be more explosive.
• It is not unanimous whether this lowers
  compressive strength.

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Postfailure Inspection

• Visual inspection of the break can reveal
• poor bonding to coarse aggregate
• segregation
• a porous and fragile matrix
• high air content
• soft coarse aggregate

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Conclusions
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• Thank you
• Questions?